The present invention relates to an intermediate transfer member used for an electrostatic recording process in an electrophotographic device or electrostatic recording device such as a copying machine or printer. A toner image, which is formed by supplying a developer on the surface of an image forming body such as a latent image support holding an electrostatic latent image, is once transferred and held on the surface of the intermediate transfer body and is then transferred on a recording medium such as a paper sheet. The invention also relates to a manufacturing method of an intermediate transfer belt as one example of the intermediate transfer body and to an intermediate transfer device using the intermediate transfer body.
In an electrostatic recording process using a copying machine, printer or the like, there has been adopted a printing method including the steps of uniformly electrifying the surface of a photosensitive body (latent image support), forming an electrostatic latent image on the photosensitive body by projecting light from an optical system on the photosensitive body to erase electrification of a portion where the light is irradiated, supplying a toner to the electrostatic latent image to form a toner image by electrostatic adhesion of the toner, and transferring the toner image on a recording medium such as a paper sheet, OHP, or photographic paper.
In color printing using a color printer or a color copying machine, the above printing process is basically adopted. However, for color printing, since a color tone is reproduced using four kinds of toners corresponding to four colors (magenta, yellow, cyan and black), there are required steps of obtaining a necessary color tone by superimposing these toners at a specific ratio. To effectively achieve these steps, there have been proposed various methods.
As a first method, there is known a multiple developing method in which, to visualize an electrostatic latent image formed on a photosensitive body by supplying toners, development is performed by a manner similar to that for monochromatic printing, that is, by sequentially superimposing toners of four colors (magenta, yellow, cyan and black) to form a color toner image on the photosensitive body. This method allows the printing apparatus to be made relatively compact; however, it is disadvantageous in that control of color gradation is very difficult and thereby a high quality image cannot be obtained.
As a second method, there is known a tandem method using four photosensitive drums aligned in line. In this method, four latent images formed on these photosensitive drums are developed using toners of four colors (magenta, yellow, cyan and black) to form four toner images (magenta toner image, yellow toner image, cyan toner image, and black toner image), and the toner images are sequentially transferred on a recording medium such as a paper sheet in a superimposing manner, thereby reproducing a color image thereon. This method is advantageous in that a desirable image can be obtained; however, it is disadvantageous in that the printing apparatus has the four photosensitive drums aligned in line, each being additionally provided with an electrifying mechanism and a developing mechanism and thereby it is enlarged in size and also increased in cost.
As a third method, there is known a transfer drum method using a transfer drum around which a recording medium such as a paper sheet is wound. Such a transfer drum revolves on its axis four times, and toner images of four colors (magenta, yellow, cyan, and black) formed on photosensitive bodies are sequentially transferred on the recording medium for each revolution of the transfer drum, to thereby reproduce a color image thereon. This method is advantageous in that a relatively high quality image can be obtained; however, it is disadvantageous in that there is a difficulty in winding a thick medium such as a post card, that is, there is a limitation to the kind of the recording medium.
In addition to the above multiple developing method, tandem method, and transfer drum method, there has been proposed an intermediate transfer method for ensuring a high quality image without enlargement of the size of the apparatus and also without limitation to the kind of the recording medium.
To be more specific, the intermediate transfer method is carried out by forming toner images of four colors (magenta, yellow, cyan, and black) on four photosensitive bodies disposed around an intermediate transfer member such as a drum or a belt, sequentially transferring the four toner images from the four photosensitive bodies onto the surface of the intermediate transfer body to form a color image on the intermediate transfer member, and transferring the color image on a recording medium such as a paper sheet. In this method, since color gradation is adjusted by superimposing toner images of four colors, a high quality image can be obtained. Also, since the photosensitive bodies are not required to be aligned in line like the tandem method, the size of the apparatus is not enlarged. Further, since a recording medium is not required to be wound around a drum, there is no limitation to the kind of the recording medium.
Such an image forming apparatus for forming a color image by the intermediate transfer method is shown in FIGS. 1 and 2, wherein FIG. 1 shows a type using a belt-shaped intermediate transfer member, and FIG. 2 shows a type using a drum-shaped intermediate transfer member.
Referring to FIGS. 1 and 2, reference numeral 1 indicates a drum-shaped photosensitive body which revolves in the direction shown by an arrow. The photosensitive body 1 is electrified by a primary electrifier 2, and is subjected to image exposure 3 for erasing electrification of an exposed portion. Thus, an electrostatic latent image corresponding to a first color component is formed on the photosensitive body 1. The electrostatic latent image is then developed with a magenta toner M as a first color toner using a developer 41 to form a magenta toner image as a first color image on the photosensitive body 1. The toner image is transferred on an intermediate transfer belt 20a (FIG. 1) or an intermediate transfer drum 20b (FIG. 2) (hereinafter, referred to as "an intermediate transfer member 20a or 20b") rotating in a state being in contact with the photosensitive body 1. In this case, the transfer of the image from the photosensitive body 1 to the intermediate transfer member 20a or 20b is performed by applying a primary transfer bias from a power supply 61 to the intermediate transfer member 20a or 20b at a nip portion between the photosensitive body 1 and the intermediate transfer member 20a or 20b. After the magenta toner image as the first color image is transferred on the intermediate transfer member 20a or 20b, the surface of the photosensitive body 1 is cleaned using a cleaning device 14. The first development/transfer operation using the photosensitive body 1 is thus completed. Thereafter, the photosensitive body revolves on its axis three times, and a cyan toner image as a second color image, a yellow toner image as a third color image, and a black toner image as a fourth color image are sequentially formed on the photosensitive body 1 using developers 42, 43 and 44 for each revolution of the photosensitive body 1. The four toner images are sequentially transferred on the intermediate transfer member 20a or 20b in a superimposing manner for each revolution, to form a synthetic color toner image corresponding to the target color image on the intermediate transfer member 20a or 20b. It is to be noted that in the apparatus shown in FIG. 1, the developers 41 to 44 are sequentially exchanged for each revolution of the s photosensitive body 1 to sequentially perform development by the magenta toner M, cyan toner C, yellow toner Y, and black toner B.
Next, a transfer roller 25 is abutted on the intermediate transfer member 20a or 20b on which the above synthetic color toner image is formed, and a recording medium 24 such as a paper sheet is fed from a paper cassette 9 into a nip portion therebetween. At the same time, a second transfer bias is applied from a power supply 29 to the transfer roller 25 so that the synthetic color image is transferred from the intermediate transfer member 20a or 20b to the recording medium 24 and is thermally fixed thereon as the final image at state 15. After the synthetic color image is transferred to the recording medium 24, the toner remaining on the surface of the intermediate transfer member 20a or 20b is removed by the cleaning device 35, and thereby the intermediate transfer member 20a or 20b is returned to the initial state to ready for the next image formation.
In the image formation by such an intermediate transfer method, however, the transfer must be repeated twice, that is, the first transfer of a toner image from the photosensitive body 1 to the intermediate transfer member 20a or 20b and the second transfer of a toner image from the intermediate transfer member 20a or 20b to the recording medium 24 must be performed, as a result of which there may occur a problem, particularly, upon the second transfer of the toner image from the intermediate transfer member 20a or 20b to the recording medium 24. The reason is that, along with repeating of printing, toner is possibly stuck and fused on the intermediate transfer member 20a or 20b, leading to reduction in efficiency of transfer to the recording medium 24 or difficulty in accurately transferring a toner image from the photosensitive body 1 to the intermediate transfer member 20a or 20b due to the presence of the toner stuck on the intermediate transfer member 20a or 20b.
Here, in the image forming apparatus using the intermediate transfer belt 20a shown in FIG. 1, as shown in the figure, the intermediate transfer belt 20a is generally disposed between the photosensitive drum 1 and the recording medium 24 in a state being wound around a plurality of (four pieces in the figure) rotating rollers 5 including at least one drive roller, and is circularly driven by the drive roller. In this case, to prevent slip-off or positional offset of the intermediate transfer belt 20a from each rotating roller 5, as shown in FIG. 5, a projecting portion 51 continuously extending in the rotational direction of the belt is integrally formed on the back surface side of the intermediate transfer belt 20a. Thus, the intermediate transfer belt 20a is circularly driven in a state in which the projecting portion 51 is fitted in a recessed portion (not shown) circumferentially provided in the surface of the drive roller among the rotating rollers 5.
The intermediate transfer belt 20a used for the prior art intermediate transfer mechanism, however, exhibits, after use for a long-period of time, the following disadvantages:
(1) The intermediate transfer belt 20a may slip off from the rotating rollers 5 or offset in its rotating path due to wear, deformation or the like of the projecting portion 51, resulting in unevenness of color of the obtained image; PA1 (2) The particles produced by wear of the projecting portion 51 may exert as adverse effect on the apparatus; and PA1 (3) The wear of the projecting portion 51 may cause noise during driving of the intermediate transfer belt 20a.
Incidentally, the intermediate transfer belt 20a used for the image forming apparatus in accordance with the intermediate transfer method is generally manufactured by winding a sheet made from a resin or rubber around the outer periphery of a cylindrical mold and vulcanizing the sheet.
The intermediate transfer belt thus manufactured in accordance with the prior art method, however, tends to cause a variation in peripheral length after vulcanizing/forming of the belt. In other words, the prior art method fails to stably obtain a belt being excellent in dimensional accuracy. Also, when the intermediate transfer belt manufactured by the prior art method is stretchingly wound around the rotating rollers 5 including the drive roller, there occurs a variation in elongation, which may obstruct normal rotation of the belt. Further, even if the belt can be normally rotated at the initial state after being stretchingly wound around the rollers, it is possibly elongated with an elapsed time, which may obstruct normal rotation of the belt.